With reference to FIG. 1, a ducted fan gas turbine engine is generally indicated at 10 and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, an intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
The high, medium and low pressure turbines 16, 17 and 18 each typically comprise a number of stages, each stage formed of a row of stator (nozzle guide) vanes and a row of rotating blades. The stator vanes are typically aerofoil-shaped and act to channel the hot gases coming from the combustion equipment 15 onto the rotating blades of the turbines 16, 17 and 18.
The stator vanes are typically formed of a composite material such as fibre-reinforced plastics material and the leading edge of each vane is typically protected from erosion and wear by bonding an electroformed metallic shield onto the composite vane to form a metallic leading edge.
Electroforming is a process that can be used to form thin metallic elements with complex curvature using electro-deposition of metal onto a mandrel. Once the deposited metal has been built up to the required thickness, the mandrel is removed to free the metallic element.
Problems arise when attempting to use electroforming to create a metallic element having a sharp, well-defined edge. In order to form such a sharp edge, the mandrel must include a corresponding sharp edge and this can be difficult to manufacture and maintain.
Furthermore, a high charge concentration tends to form at the sharp edge during electro-deposition which leads to an increase in thickness of the deposited metal. This, in turn, leads to an undesirable increase in the radius of curvature at the edge of the metallic element i.e. the edge is not sufficiently sharp/well-defined.
This effect can be reduced using careful shielding of the mandrel to prevent excessive deposition but the resulting metallic element tends to be thin at the sharp edge with a sharp internal groove (caused by the sharp edge on the mandrel). This is undesirable from a mechanical viewpoint.
There is the need for a method of electroforming a metallic element such as a shield for an aerofoil component (e.g. a stator vane) where the resulting metallic element has a sharp, well defined edge (with a small radius of curvature) without compromising mechanical strength.